This was ariadne's contribution to cupcake camp; I am posting it on her behalf. This was one of the best frostings there... the cake was good, but the coconut buttercream was AMAZING. You could probably put it on a rock and it would be declared the best! cupcake! EVER!
1 cup boiling-hot water
3/4 cup unsweetened cocoa powder
1/2 cup whole milk
1 teaspoon vanilla
2 cups all-purpose flour
1 1/2 teaspoons baking soda
Rounded 1/2 teaspoon salt
2 sticks (1 cup) unsalted butter, softened
2 cups packed dark brown sugar
4 large eggs at room temperature for 30 minutes
Put oven rack in middle position and preheat oven to 350°F. Line cupcake pans with paper liners.
Whisk together hot water and cocoa powder in a bowl until smooth, then whisk in milk and vanilla.
Whisk together flour, baking soda, and salt in another bowl.
Beat together butter and brown sugar in a large bowl with an electric mixer at medium-high speed until pale and fluffy, 3 to 5 minutes. Add eggs 1 at a time, beating well after each addition. Reduce speed to low and add flour and cocoa mixtures alternately in batches, beginning and ending with flour mixture (batter may look curdled).
Fill cupcake liners to about 3/4 full. Bake until a wooden pick or skewer comes out clean and edges of cake begin to pull away from sides of pans, 15 to 20 minutes total. Cool in pans on racks 10 minutes, then remove from pans, and cool completely.
4 large egg whites at room temperature (or use pasteurized egg whites from a carton)
1/2 teaspoon salt
1 1/4 cups sugar
1/2 cup water
1/2 teaspoon fresh lemon juice
3 sticks (1 1/2 cups) unsalted butter, cut into tablespoon pieces and softened
1 1/2 teaspoons coconut extract
Special equipment: a candy thermometer
Combine egg whites and salt in bowl of a standing electric mixer or other large bowl.
Stir together sugar and water in a small heavy saucepan and bring to a boil over moderately high heat, stirring until sugar is dissolved and washing down side of pan with a pastry brush dipped in cold water. When syrup reaches a boil, start beating whites with electric mixer at medium-high speed. Once whites are frothy, add lemon juice and beat at medium speed until they just hold soft peaks. (Do not beat again until sugar syrup is ready—see below.)
Meanwhile, put thermometer into sugar syrup and continue boiling, without stirring, until it reaches soft-ball stage (238–242°F). Immediately remove from heat and slowly pour hot syrup in a thin stream down side of bowl into egg whites, beating constantly at high speed. Beat meringue, scraping down bowl with a rubber spatula, until meringue is cool to the touch, about 6 minutes. (It's important that meringue is fully cooled before proceeding.) - it took mine at least 15 to 20 minutes to cool enough. Don’t use a hand mixer – it will die, believe me.
With mixer at medium speed, gradually add butter 1 piece at a time, beating well after each addition until incorporated. (If meringue is too warm and buttercream looks soupy after some butter is added, chill bottom of bowl in a larger bowl filled with ice and cold water for a few seconds before continuing to beat in remaining butter.) Continue beating until buttercream is smooth. (Mixture may look curdled before all butter is added, but will come back together before beating is finished.) Add coconut extract and beat 1 minute more.
•Buttercream can be made 1 week ahead and chilled, covered, or 1 month ahead and frozen. Bring to room temperature (do not use a microwave) and beat with an electric mixer before using.